The invention relates to a magnetic-tape deck for cassettes containing a magnetic tape which can be transported between two reels, such as the well-known VCR, that is a deck which comprises: a chassis; at least one magnetic head which is movable in a circular path; a drum having a circumferential surface for guiding a length of magnetic tape which is wrapped around a part of the drum and which cooperates with the magnetic head; a playing-loop device for withdrawing a length of magnetic tape from the cassette and subseqyently wrapping it around the drum to form a playing loop in a "play" mode of operation of the deck, the drum being situated in the central part and the reels at the ends of the playing loop, and a fast winding mechanism having an element for draining static electricity from the tape.
In the decks with which the invention is used, playing-loop device comprises first and second playing-loop guides for guiding the playing loop in guide positions upstream and downstream respectively of the drum, and first and second playing-loop-guide supports which are movable between an inoperative position, in which the playing-loop guides are disposed in the cassette, and an operative position, in which the playing-loop guides are in the upstream and downstream guide positions.
The fast-winding mechanism, for fast winding of the magnetic tape between the reels in the cassette during a "fast-forward" or "fast reverse" mode of operation of the deck, comprises a fast-winding-loop device for withdrawing a length of magnetic tape from the cassette and subsequently guiding it to form a fast-winding loop outside the cassette. The fast-winding-loop device comprises at least one fast-winding-loop guide; a fast-winding-loop-guide support for the fast-winding-loop-guide, which support is movable between an inward position, in which the fast-winding-loop guide is disposed inside the cassette at the back of the magnetic tape, and a fast-winding position, in which the fast-winding-loop guide is disposed outside the cassette; and a positioning device for moving the fast-winding-loop guide support between the said positions.
The deck also includes at least one draining elements of an electrically conductive material which during fast-winding is in contact with the front of the length of magnetic tape that has been withdrawn from the cassette to form a fast-winding loop, and which drains static electricity from the front of the magnetic tape. As used in this specification a "loop" in the tape is defined as a stretch of tape from one tape spool to another tape spool.
Magnetic-tape decks which are substantially of this type are in use in large numbers in home-entertainment video recorders; see, for example, U.S. Pat. No. 3,939,491. The advent of the video recorder as an apparatus suitable for domestic use has been accelerated by a number of developments, such as the availability of cassettes, improved and automatic playing-loop devices, improved magnetic tapes which in conjunctions with improved magnetic heads make it possible to obtain a substantially higher information density and thus a substantial reduction in tape consumption, and the progress in micro-electronics which enables extensive and intricate circuitry to be manufactured at an acceptable cost.
Mainly two different types of playing-loop devices are used in current video recorders. Perhaps most frequently used are playing-loop devices in which the magnetic tape is formed into a roughly M-shaped loop, the co-called "M-loop wrap" devices. This nearly always concerns helical-scan magnetic-tape recorders in which the magnetic tape is wrapped around the drum in a helical path. In helical-scan magnetic-tape recorders the M-shape of the loop is especially apparent in a plan view of the apparatus. All the parts of the M are not situated in one plane because the magnetic tape is wrapped helically around the drum. The magnetic tape is wrapped around the drum starting from a leading-end point at one side of the drum towards an axially offset trailing-end point at the other side of the drum.
Another frequently used playing-loop device is the so-called "C-loop" device in which the magnetic tape is wrapped around the drum substantially along one side in a more or less circular path, so that the drum is disposed in the centre of the C. The magnetic tape extends from one reel along the outside of the C, is then diverted and returns to the other reel via the inside, along the drum.
It has been found that in magnetic-tape decks problems may occur as a result of static electricity produced during unwinding, transport and taking-up of the magnetic tape. These problems occur mainly during fast winding of the magnetic tape, because the degree of electrostatic charging depends on the speed of transport of the magnetic tape and because in the fast winding mode, in contradistinction to the recording and/or play-back modes, the magnetizable front side of the magnetic tape is generally not in contact with the drum or with other metal tape guides.
The formation of electrostatic charges in magnetic tapes whose magnetizable front layer is not electrically conductive is described in, for example, U.S. Pat. No. 3,601,654 (herewith incorporated by reference). When two layers which are constantly in contact with each other are separated from one another static charge is formed on the surfaces which were previously in contact. This effect is most pronounced when the materials are poor elecrical conductors, because the charge is retained longer by such materials.
When a nonconductive magnetic tape is unwound from a reel, the back of the outermost turn of the roll is separated from the front of the next turn, so that static electricity is generated in an unpredictable pattern. As more magnetic tape is wound onto the other reel, the charge at the back of the magnetic tape would cancel the opposite charge at the front of the magnetic tape if the diameters of the two rolls were constantly equal to each other. The electrostatic-charge pattern at the back would then always correspond to the pattern at the front of the magnetic tape. However, since this situation occurs only momentarily, halfway along the length of the magnetic tape, a residual static-charge pattern will persist on the magnetic tape after it has been transferred from one reel to the other.
Any static charge on the magnetic tape will cause problems. Apart from the fact that it causes the magnetic tape to stick, which may lead to an irregular tape transport and possible even to folding and jamming of the magnetic tape in the cassette, the surface of the magnetic tape will attract dust particles. These particles may impair the correct operation of the magnetic heads and may also contribute to wear of the magnetic heads. In addition, the static charge may also lead to the magnetic tape being incorrectly wound onto the reels.
In some cases of incorrect winding, folds may form in the magnetic tape inside the cassette to the extent that the cassette becomes unserviceable. At the least, creases may be formed locally in the magnetic tape so that it is no longer suitable for recording or playback. It will be obvious that these problems occur mainly in the case of magnetic tapes whose materials at the back and at the front give rise to the occurrence of considerable electrostatic charges and in the case of thin magnetic tapes. Modern magnetic tapes are very thin, their thickness being of the order of 15 microns. There is a tendency to reduce the thickness of magnetic tapes even further, to thicknesses of the order of 6 to 10 microns.
It is assumed that the degree of electrostatic charging of the magnetic tapes is proportional to the speed with which the surfaces of the two layers are separated from each other, under specific atmospheric conditions. Consequently the problem is mainly of significance during fast winding of the magnetic tape.
The phenomenon of the formation of electrostatic charges in magnetic tapes having a magnetisable front layer which is electrically well conductive, for example, a vacuum-deposited metal coating, is described in, for example, U.S. Pat. No. 4,285,020 (herewith incorporated by reference). The static charge does not remain at one location but distrubutes itself over the length of the tape. The charge may, for example, be caused by tribo-electrification as a result of friction. Eventually the electric potential may become so high that flash-over occurs between the magnetiseable front side and metal parts in the vicinity of the magnetic tape.
The prior art provides various solutions for draining static electricity from the front side of a magnetic tape in a cassette. For example, the afore-mentioned U.S. Pat. No. 3,601,654 describes a modified standard audio cassette comprising draining elements in the form of electrically conductive tape guides inside the cassette, which elements are in contact with the front of the magnetic tape and which are electrically connected to other tape guides which are in contact with the back of the magnetic tape. The casette housing may be metallised or made of metal, the cassette being grounded through the magnetic-tape apparatus during use of the cassette on a magnetic-tape apparatus. The afore-mentioned U.S. Pat. No. 4,285,020 also relates to a magnetic-tape cassette in which static electricity can be drained from the front side of the magnetic tape to parts of a magnetic-tape apparatus by means of an electrically conductive draining element. Such methods therefore necessitate the use of draining elements in the cassette which cooperate with the front of the magnetic tape. This makes the construction of the cassette more intricate and it leads to a higher cost-price of the cassette. Especially in the case of cassettes for use in magnetic-tape decks of the type mentioned in the opening paragraph, additional tape guides in the cassette, apart from the previously mentioned undesirable effects, are undesirable because they adversely affect the uniformity of the transport speed of the magnetic tape during the recording and/or reproduction of signals and also the correct guidance of the magnetic tape relative to the tape guides and loop guides present on the deck. This is because the tape guides in the cassette have been aligned accurately relative to the cassette housing and not relative to the deck.
From the afore-mentioned U.S. Pat. No. 3,939,491 a video-recorder deck equipped with a C-loop device is known, comprising a fast-winding-loop device having two movable fast-winding-loop-guide supports with fast-winding-loop guides which engage behind the magnetic tape in the cassette. By means of the fast-winding-loop guides the magnetic tape can be withdrawn from the cassette over some distance, enabling fast winding to be effected in this situation. The length of magnetic tape withdrawn from the cassette to form a fast-winding loop is led from one reel to the other reel tangentially to the drum and over a pair of stationary tape guides, which are arranged on the deck and which cooperate with the magnetisable front of the magnetic tape. A stationary magnetic head is capable of detecting the presence of pulses on the magnetic tape and transferring these pulses to a counting circuit for measuring the length of the magnetic tape thus transported. Such a video recorder permits fast winding with a part of the magnetic tape outside the cassette, the front of this part of the tape being in contact with electrically conductive tape guides which are capable of draining the static electricity.
The fast-winding-loop device known from U.S. Pat. No. 3,939,491 for withdrawing the magnetic tape from the cassette and subsequently guiding this tape outside the cassette during fast winding is not suitable for use in magnetic tape decks comprising an M-loop device. Stationary draining elements on the chassis of the deck in the case of the known C-loop device do not impede the formation of a loop of magnetic tape around the drum, because the playing-loop guides move along one side of the drum. In an M-loop device this presents more problems becuse the playing-loop guides then move on both sides of the drum, so that stationary draining elements, at least in the case of a compact construction, may form an obstacle. A further disadvantage of this known magnetic-tape apparatus is that a comparatively large distance is required between the cassette and the drum. The present tendency, at least in video recorders, is to make the deck as compact as possible. This is particularly important in decks for portable video recorders, especially if the deck is to be incorporated into an electronic camera.